The SCART interface has been widely utilized for the video/audio signal transmission in TV sets or players. For example, the SCART interface is a standardized interface that must be implemented in the TV sets and VCR players for sale in European marketplace. FIG. 1A and FIG. 1B are diagrams illustrating the SCART interface and its connectors. FIG. 1A demonstrates a SCART connector 10 that has 21 connecting pins that includes pins for audio/video signals and pins designated for conveying information that controls the transmission of audio/video signals. FIG. 1B demonstrates a SCART socket 11 that has connecting holes for adapting the corresponding connecting pins on the SCART connector 10.
Other than TV sets and players, the SCART interface has been also pervasively equipped in audio/video playback devices or signal conversion devices such as video game consoles, disc players, set-top boxes, or video signal converters. As shown in FIG. 1C, for example, a set-top box 12 can be used to integrate audio/video signals from various sources for the displaying on a TV set through a plurality of SCART sockets 121 and 122. Users can allow the audio/video signal transmission among SCART sockets by connecting them by a SCART cable. For example, as shown in FIG. 1D, a SCART-to-SCART connection can be made by connecting connectors 101 and 102 disposed at two ends of a SCART cable 100 to a SCART socket 141 of a TV set 14 and a SCART socket 161 of a VCR player 16 (or a audio/video playback device such as a disc player, video game console, or set-top boxes), respectively.
Among the 21 connecting pins of a SCART connector, one of the pins, the Pin 8, has been assigned to convey the information to control the aspect ratio of the displayed image. It allows a TV set to determine the proper aspect ration between different aspect rations including the traditional 4:3 screen and 16:9 widescreen. The TV set that connected with a player via SCART interface receives the controlling signal from the player and determines the aspect ratio the player is using according to EN50049-1 and IEC 60933 industry standards. These standards defined the relationship between the voltage level of the controlling signal on the pin 8 and the aspect ratio that is currently assigned such that the TV set can display the image with properly settings. The detailed definition of the signal on the pin 8, as shown in FIG. 2A, is: if the voltage level is between 9.5V and 12V, the assigned aspect ration is 4:3; if the voltage level is between 4.5V and 7V, the assigned aspect ration is 16:9; and if the voltage level is between 0V and 2V, there is no image signal (inactive source) or no specific aspect ration assigned.
To determining the voltage level of the input controlling signal and the aspect ratio it represents, the simplest and most feasible way is to compare the input controlling signal by multiple comparators and translate the comparing results into digital output that can be further processed by logic circuits. Conventionally, in a SCART interface there were two comparators connected to a SCART socket, and the following logic circuit can determine the voltage level of the input controlling signal according to the combination of the two logic output of the comparators.
FIG. 2B is a circuit diagram of the implementation of the comparators 21 and 22 in the SCART interface 20. The negative input of each comparator 21 and 22 is connected to a reference voltage, while the positive input is connected to the input signal. According to the definition illustrated in FIG. 2A, the reference voltages can be set as 8.9V and 4V for comparators 21 and 22, respectively. When the input signal is with voltage level between 9.5V to 12V, both comparators output logical “1.” Similarly, if the input signal is with voltage level between 4.5V to 7V, the comparators 21 and 22 will output logical “0” and “1,” respectively. And two logical “0s” will be output if the input signal is with voltage level between 0V to 2V. Therefore, the following logic circuit can learn the assigned aspect ration according these logic outputs. For example, when the output is “11,” it means the 4:3 screen; when the output is “01,” it means the 16:9 screen; and when the output is “00,” it means the “inactive” setting.
However, the conventional implementation by using two comparators usually accompanies with higher manufacturing costs. And a low-cost solution with a high precision for detecting the voltage level of the input signal becomes an important issue to be resolved.